\renewcommand{\thefigure}{\arabic{figure}}

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\begin{figure}[htbp] \vskip 0.0cm \centering
\includegraphics[width=8.5cm]{fig_decay_chain.eps}
\caption{(a) Radioactive decay of radon and its progeny. 
The half-life, decay type and decay energy of each species are also listed.
(b) The simplified decay chain considered in our simulations.
\label{fig:decay_chain} }
\end{figure}

\begin{figure}[htbp] \vskip 0.0cm \centering
\includegraphics[width=8cm, angle=0]{fig_emis.eps}
\caption{(a)--(c): Radon emission setups considered in this study.
The quantity shown is the annual mean atom number flux 
(unit: atom~cm$^{-2}$~s$^{-1}$). (d): Data source of the merged 
emission map shown in panel (c). 
S1 stands for \citet{schery:1998} (scaled), S2 for \citet{schery:2004},
G for \citet{griffiths:2010}, 
Z for \citet{zhuo:2008}, and S3 for \citet{szegvary:2007}.
\label{fig:emis} }
\end{figure}

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\begin{figure*}[htbp] \vskip 0.0cm \centering
\includegraphics[width=8cm, angle=90]{fig_location.eps}
\caption{Location of surface radon measurements used in this study. 
See table~\ref{tab:sites} for further information of the sites.  
Colors indicate observational sites in the five different regions 
that are analysed separately in Fig.~\ref{fig:scatter_region}. 
Enlarged maps for Europe, East Asia, and USA are shown in the supplementary material. 
\label{fig:location} }
\end{figure*}

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\begin{figure*}[htbp] \vskip 0.0cm \centering
\includegraphics[width=15cm]{fig_scatter.eps}
\caption{ 
Scatter plots of the simulated and measured monthly or seasonal mean 
surface radon concentration~(mBq~m$^{-3}$~STP) 
at the 51 sites listed in Tab.~\ref{tab:sites} and shown in Fig.~\ref{fig:location}. 
The three columns correspond to simulations using the WCRP1995 recommended 
radon emission~(left), the SW1998 emission maps scaled by a factor of 1/1.6~(middle),
and the new emission maps prepared during this study.
The first row contains results of all months/seasons (534 samples);
The second row shows the 129 winter samples~(DJF in the
Northern Hemisphere, JJA in the Southern Hemisphere), 
and the third row shows only the 135 summer samples~(JJA in the Northern Hemisphere, 
DJF in the Southern Hemisphere). 
The dashed lines indicate the range within a factor of 2 of the measurements. 
Also shown in each panel are the percentage of samples within this range~(the P2 values) 
and the correlation coefficients between simulation and observation~(the R values).
\label{fig:scatter} }
\end{figure*}

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\begin{figure*}[htbp] \vskip 0.0cm \centering
\includegraphics[width=15cm]{fig_scatter_region.eps}
\caption{
Same as Fig.~\ref{fig:scatter}, but focusing on different regions. 
The four rows show results in (from top to bottom) Europe, China, the United States 
and South America. All seasons are included. 
The marker colors are consistent with Fig.~\ref{fig:location}. 
\label{fig:scatter_region} }
\end{figure*}

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\begin{figure*}[htbp] \vskip 0.0cm \centering
\includegraphics[width=15cm]{fig_scatter_clim.eps}
\caption{
Same as Fig.~\ref{fig:scatter}, but for the climatological simulations 
without nudging. 
\label{fig:scatter_no-nudging} }
\end{figure*}

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\begin{figure*}[htbp] \vskip 0.0cm \centering
\includegraphics[width=16cm]{fig_box_eur.ps}
\vspace{8pt}
\caption{Box plots showing the simulated and observed distribution of 
surface radon concentration at six sites in Europe.
The two whiskers of each box denote the 10th (lower) and 90th (upper) percentiles. 
Hinges from bottom to top are the 25th, 50th, and 75th  
percentiles respectively. Seasonal and annual means are indicated by dots. 
The gray areas indicate magnitude of the equivalent radon concentration 
that would lead to the same ionization rate as caused by galactic cosmic rays.
The lower and upper boundaries of the gray areas correspond to 
the 10th and 90th percentiles, respectively. See paragraph 3 of 
Section~\ref{subsec:box} for further details. 
\label{fig:box_eur} }
\end{figure*}

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\begin{figure*}[htbp] \vskip 0.0cm \centering
\includegraphics[width=16cm]{fig_box_chn.ps}
\vspace{15pt}
\caption{Same as Fig.\ref{fig:box_eur} but for six city sites in China. 
\label{fig:box_chn} }
\end{figure*}

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\begin{figure}[htbp] \vskip 0.0cm \centering
%\includegraphics[width=14cm]{fig_box_usa.ps}
\includegraphics[width=8cm]{fig_box_usa.ps}
\vspace{8pt}
\caption{Same as Fig.\ref{fig:box_eur} but for three sites 
in the United States. 
\label{fig:box_usa} }
\end{figure}

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\begin{figure*}[htbp] \vskip 0.0cm \centering
\includegraphics[width=16cm]{fig_box_ocn.ps}
\vspace{8pt}
\caption{Same as Fig.\ref{fig:box_eur} but for coastal and oceanic sites. 
\label{fig:box_ocn} }
\end{figure*}

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\begin{figure*}[htbp] \vskip 0.0cm \centering
\includegraphics[width=17cm]{fig_surf.eps}
\caption{Simulated annual and seasonal mean near-surface 
ionization rate induced by radon decay series (IPRR, unit: cm$^{-3}$~s$^{-1}$).
\label{fig:surf} }
\end{figure*}


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\begin{figure*}[htbp] \vskip 0.0cm \centering
\includegraphics[width=16cm]{fig_zonal_contrib.eps}
\caption{
Left column: simulated zonal mean ionization rate over the continents caused by 
the radioactive decay of radon and its progeny (IPRR, unit: cm$^{-3}$~s$^{-1}$);
Middle column: as in the left column but caused by 
galactic cosmic rays (IPRC, unit: cm$^{-3}$~s$^{-1}$); 
Right column: the contribution of radon and its progeny to the total
(IPRR + IPRC) ionization rate. All panels correspond to the simulation 
performed with the merged emission map.
\label{fig:zonal_contrib} }
\end{figure*}

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\begin{figure}[htbp] \vskip 0.0cm \centering
\includegraphics[width=8cm]{fig_latslice.eps}
\caption{Height-longitude cross section of the 20$^{\circ}$N--50$^{\circ}$N 
mean ionization rate over land caused by radon and its progeny. 
The results are obtained 
using the newly merged radon emission map.
%\caption{Simulated 20N--50N meridional mean~(colored regions) and 
%         90$^{th}$ percentile~(contour lines) ionization rate induced 
%         by radon decay series~(IPRR) over land areas. 
\label{fig:latslice} }
\end{figure}


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\begin{figure*}[htbp] \vskip 0.0cm \centering
\includegraphics[width=16cm]{fig_jointPDF.eps}
\caption{Joint (bivariate) probability density distribution (PDF) 
of air temperature and radon-related ionization rate (IPRR) in different regions:  
(a) China (20$^{\circ}$N--50$^{\circ}$N, 75$^{\circ}$E--120$^{\circ}$E); 
(b) Europe (40$^{\circ}$N--75$^{\circ}$N,10$^{\circ}$W--40$^{\circ}$E);   
(c) USA (30$^{\circ}$N--50$^{\circ}$N,120$^{\circ}$W--70$^{\circ}$W); 
(d)-(f) Russia (50$^{\circ}$N--80$^{\circ}$N, 40$^{\circ}$E--180$^{\circ}$E).
These regions are indicated by dashed black frames in Fig.~\ref{fig:surf}.
Labels next to the color bar are intensities of the PDF (unit:\%). 
The PDFs are computed for the near-surface from the 3-hourly model output 
in winter months (DJF). Marginal area with white color indicate missing values.  
Note that scales of the temperature coordinate in (a-c) are not the same 
as those in (d-f). 
\label{fig:jointPDF} }
\end{figure*}

